Multiplex pulse modulation system



y 2 1946- w. A. s. BUTEMENT ETAL 2,403,210

MULTIPLEX PULSE MODULATION SYSTEM Filed Feb. 14; 1944 4 Sheets- Sheet 1 MRKER A U m A A Fga.

2,10%, a. 1 u tor az/m-ak.

Attorr rey y 2, 1946- w. A. s. BUTEMENT ETAL' 2,403,210.

I MULTIPLEX PULSE MODULATION SYSTEM Filed Feb. 14, 1944 4 sheet -sheet 3 & In nior ina Attorngy 4 Sheets-Sheet 4 w. A. s. BUTEMENT ETAL HULTIPLEX PULSE MODULATION SYSTEM Filed Feb. 14, 1944 July 2, 1946.

l llnllllnl-llllvlllllllll Patented July 2, 1946 MULTIPLEX PULSE MODULATION SYSTEM William Alan Stewart Butement and Alan John Henry Oxford, London, England Application February 14, 1944, Serial No. 522,350

In Great Britain December 4, 1942 The present invention relates to electrical signalling systems of the type in which the signals, for example speech signals, are transmitted as fixed-amplitude pulses of radio frequency occurring at a fixed repetition frequency higher than the highest signal frequency it is desired to transmit and having a duration which varies in accordance with the instantaneous amplitude of the signals. Such pulses will be hereinafter referred to as variable-width pulses.

It is an object of the present invention to provide a multi-channel signalling system employing a single radio or cable link between transmitter and receiver in which this method of si nal transmission is utilized.

A further object of the invention is to provide a multi-channel system in which the equipment necessary is lighter and smaller and is simpler and cheaper to construct than that of multichannel systems hitherto proposed, and in which the power available per channel is greater and the eifect of noise or-fading less than in previouslyproposed systems.

According to the present invention there is provided an electrical signalling system of the type described wherein signals from a number of channels are transmitted in the form of a train of variable-width pulses for each channel over a single link to a receiving point together with a train of marker" pulses of a fixed width ap- 9 Claims. (01. 179-15) v tial to the control and suppressor grids of a pen; tode valve the interlaced trains of positive pulses being applied to one of these grids, the other grid preciably greater than the maximum width of the signal pulses, the pulses in each train having the same repetition frequency but being displaced in time relative to one another so that they can be interlaced for transmission over the single link, and wherein (at the receiving point) the marker pulses are separated from the signal pulses and are utilized to generate for each channel a train of "stroboscope pulses of fixed width and of the same repetition frequency and timing as the signal pulses of that channel, the stroboscope" pulses being utilized to effect the separation of the trains of signal pulses from one another.

According to a feature of the invention trains of pulses displaced in time relative to one another are generated by applying a saw-toothed wave of the desired repetition frequency to the control grids of a number of valves having an initial grid potential which is negative with respect to the cathode potential, the sudden drop in the anode voltage of each valve which occurs each time the rising grid potential approximately equals the cathode potential being utilized to develop a pulse, whereby each valve is used in developing a separate train of pulses, the relative timing of any two trains being determined by the relative values of the initial potential difference between the cathode and control corresponding valves.

According to a further feature of the invention a particular train of variable width pulsesis selected from a number of interlaced trains of variable width pulses by applying a negative potenhaving applied thereto a train of positive strobo-- scope pulses of the same frequency and timing,-v as the train'to be selected, and of a fixed-width equal to the maximum width of the pulses to be selected, whereby the valve conducts only during the periods when a stroboscope pulse and a pulse of the train to be selected are simultaneously present,

The invention will now be described by way of example with reference to the accompanying drawings in which Figs. 1-4 illustrate in its simplest form a speech transmission system having eight speech channels and a single ultra-short wave radio link between the transmitter and receiver, whilst Figs. 5-8 are circuit diagrams and curves illustrating a preferred form of such a speech transmission system. In the drawings Fig. 1 is an explanatory diagram showing the interlacing of the pulses;

Fig. 21s a simple form oftransmitter;

Fig. 3 is a series of curves showing the ,voltages at the electrodes of the valves of Fig. 2":

Fig. 4 is a schematic diagram of simple form of receiver;

Fig. 5 shows the transmitted wave form of the preferred system;

Fig. 6 is a circuit diagram of a preferred form of pulse generating system;

Fig. 7 is a series of curves illustrating the generation of the marker pulses; and

Fig. 8 is a circuit diagram of the preferred receiving. circuit.

In the simple arrangement to be described with reference to Figs. 1-4, the pulse train for each speech channel comprises pulses recurring at a fixed repetition frequency of approximately 9100 per second, the width of the pulses varying in accordance with the speech modulation in such a way that at a maximum depth of modulation they have a duration of 8 microseconds and at a minimum depth they have a duration of 2 microseconds. The pulse train for the marker" channel, by means of which the fixed repetition frequency at which all the channels operate is congrid of the two I veyed to the receiver and there used to sort out the various spefiih channels. comprises pulses ot fixed width rec ring at the 9100 cycle repetition frequency the duration of each pulse being 14 microseconds; *The manner in which the nine l trains of pulses'are interlaced is shown in Fig. 1.

Considering one complete cycle of the repetition i -frequency, which-has a duration of 109.5 microseconds, thefirstld microseconds are occupied by a marker pulse, This is followed by a gap of 3% microseconds, and the succeeding period of 8 5 microseconds is available for a pulse of the first speech channel, the end of the pulse coinciding with the end of this periodv and the onset of. the

pulse varying, in accordance with the speech modulation. This is followed bya second gap of 3% microseconds, and a-second period of 8 microdare not essential to should be appreciably longer than the maximum Vi duration of thespeech pulses in order that it can be satisfactorily separated from them at the receiver. Itis also essential that the maximum 1 duration ofthe speech pulses should be limited to allow an adequate gap between consecutive pulses or maximum width and that the minimum dura- I tion otthe speechjpulses and of this gap should.

J not .be so short as to make necessary a prohibitively wide band pass for the system as a whole. In the following description of the transmitting and receiving circuits and the manner in which they operate it will be. assumed that time t=- -0 marks the beginning of each cycle of the repetition frequency and that time t=l09.5 marks the endof; each cycle, tbeing. measured in microseconds; r v

The transmitter compriseseightseries of valves for generating thespeech channel pulses (one series being provided for each channel) and a ninth series for generating the marker pulses.-

In'Fig. 2, the ninth series is indicated by the references VIA and V2A, whilst the first two only of the speech channel pulsegenerators are shown (-VlB--V3B and VIC-V30). A common output 'valve V4 is provided for allthe nine series. A

saw-toothed voltage from .the generator lis' applied tothe control grid or the first valve of 1 eachseries, the period of this voltage being that of the desired repetition frequency (curve A, Fig.

'3) The first valve in each series is normally cutoil sothat its anode voltage will remainat a .4 t=14 thus appear in the anode circuit of this second valve (curve D, Fig. 3), and these are transferred to the common output valve V4, ap-

pearing as positive pulses, or pulses of increased anode voltage, in its anode circuit (curve P,

Fig. 3);

In generating the speech pulses, the sudden drop in anode voltage which occurswhen .the Y first valve VIB, VIC, etc., of each pulse generator suddenly conducts is utilized. The time at which this occurs'obviously depends upon the cathode potential, and inorder to achieve the desired interlacing of the eight trains of pulses, the cathode of each of the first valves is connected to a different tapping on a potentiometer l, the cathode for the first generator VlB being the least and that for the eighth, the most positive. The pulses for the first speech channel are generated as follows: The cathode positive potential of the first valve VIB is held at such a value that the rising grid potential approximately equals it at t=17.5, and the sudden drop in anode voltage therefore occurs at this point (curve E,

Fig. 3). The first valve is connected through a condenser-resistance network 5, 6 to the control grid of a second valve V213, this grid being'normally held slightly positive. The time constant of this network is such that the grid of the second valve drops toa negative value at t=l'l.5 and is maintained at this value until t=25.5, when it returns to its normal positive value (curve G, Fig. 3). The anode potential of this second valve is normally at a low potential except during this negative pulse on the grid, so that a condenser I connected between the anode and earth begins to charge at the beginning of the grid pulse and is suddenly discharged at the end of it. A short saw-toothed pulse beginning at t=l7.5 and -ending at t=25.5 is thus generated by the second valve (curve I, Fig. 3). This is applied to the control grid of a modulator valve V33 having a transformer 8 in its cathode circuit to the input of which the speech potentials from channel high-value from i=0 until the rising voltage on its-grid approximately equals its cathode voltage.*

The valve then conducts and the anode voltage drops toa lower value at which it remains until the beginning of thenext cycle, at which point it suddenly rises to the cut-oil value again. This sudden rise in the anode voltage of the first valve VIA oi the marker channel pulse generatoris '..utilized for developing the marker pulses (curve B, Fig. 3). For this purpose. the first valve VIA is connectedthrough a .,resis tance -condenser network 2, 3 to the control grid of 'a" second valve V2A, the time constant of this network being so chosenthat this control grid rises to a positivev value at i=0 and is maintained at this value until t=14, when it reverts to its normal negative value (curve C, Fig. 3). Negative pulses, 0r pulses of reduced anode voltage, lasting from t=0 to lasting from 2-8 microseconds. pulses of anode current vary in width from cycle to "cycle 'sinusoidally in accordance with the No. 1 are applied. The short saw-toothed pulse of voltage applied to the grid is opposed by a steady positive voltage on the cathode which has a value such that, in the absence of speech modulation, the grid is driven positive for 5. microseconds from t=20.5 to t=25.5. Application of speech potentials'to the cathode causes the grid potential to swing in accordance therewithso that the grid is driven positive for times varying I from;2 to 8 microseconds, that is from between t= 23.5 and t=17.5 to t=25.5'microseccnds (curve K, Fig. 8). The anode of-the modulator valve is held positive except during this period when the grid is driven positive, on which occasion it is driven negative due to the surge of anode current The resulting speech modulation between the limits t=23.5 and t=l'l.5 for the commencement of the pulse and t=25.5 for the end of the pulse (curve M, Fig. 3). The pulses for the second channel are generated in exactly the same way (curve F,.H. J, L

' and vN, Fig. 3) except that the cathode potential of the first valve is chosen at a slightly higher positive value so that the sudden drop in the anode current occurs at t=.29. This results in pulses commencing from between t=35 and t -'29 and ending at t=37. The pulses for the remaining channels are successively delayed by equal amounts by appropriate choice of the cathode potentials for the first valve in each case.

The anode circuits of all the eight modulator valves are connected to the control grid of the common output valve, so that the negative pulses generated by eachot these valves are applied to the grid one after the other (curve 0, Fig. 3). Corresponding positive pulses for all the eight channels, interlaced in the required manner, thus appear in the anode circuit of the output valve (curve P, Fig. 3). The output of this valve is used to modulate an ultra-short wave radio sender 9. x

The transmitted signal is received on a normal receiver l0. (Fig. 4) of the supersonic heterodyne type having 'a pass-band wide enough to handle the narrowest pulses which may occur. The output of the second detector of the receiver will consist of the series of interlaced pulses already described. The first step is to reproduce at the receiving point the repetition frequency at which all the channels operate. This is done by applying a portion of the output: of the second detector to the arid of a valve ll, whose function is to convert the positive pulses occurring, in this output into negative pulses. It the output already consists of negative pulses this valve is not required. The negative pulses are applied to the of Fig. 3. The required pulses appear in'the' anode circuit of the second valve.

The other portion of the output from the-second detector of the receiver is fed to the control grids of eight pentode valves Pl-P8 connected in parallel. To the suppressor grid of the first oi valves Blythe potential orwhich is normally negative, are applied the first train of stroboscope pulses from thegeneratorGl. The valve passes current only during the periods when the stroboscopepulses andthe speech pulses Of channel I are simultaneously present and consequently there will app ar in the output circuit only the pulses of the first speech channel. Similarly,the second train oi stroboscope pulse irom-G2 are applied to the suppressor grid of the second pentode valve P2, sothat there will appear in the output circuit of this valve only the pulses of." the second speech channel, and so on. Thusthe, pulses of the eight speech channels have been sorted out from one another. The eight pentode valves are connected to output circuits: ol-oaj control grid of a valve l2, the cathode of which is held slightly negative, so that the valve passes anode current of saturation value except during the persistence oi! the pulses. A condenser ll connected between the node of this valve and earth will thus charge during the persistence of each pulse and will discharge suddenly at the end or the pulse: the voltage to which the condenser is. charged willdepend upon the length of the pulse. As the length of the marker pulse exceeds that of any other by at least 6 microseconds the condenser voltage will reach a greater positive value at the end or the period or application 01' a marker pulse to the grid of the valve than at the end of the period of application of any speech pulse. Thevoltages developed across the condenser are applied to the control grid of a second valve ll, the cathode potential of which is held at such a positive value that only the voltages produced by the marker pulses can cause any pamage oi anode current. The condenser voltages produced by the speech pulses are insuillcient to swing the grid positive. Consequently, pulses corresponding to the marker pulses only appear in the anode. circuit of the second valve;

these occur at the desired repetition Irequencyand are used to lock an oscillator i6 generating a saw-toothed wave substantially identical with that employed at the transmitter. By means of a. slightly modified version of the process already wave is utilized .to generate eight trains of positive pulses, which may be termed stroboscope" pulses. Thme are unmodulated, having a fixed duration 018 microseconds, and the pulses in grammatically at GI-GB in Fig. 4. Each comprises a pair of valves connected i the same way as the first two valves o! each speech channel pulse generator and operating in the manner already described and illustrated in curves E-H described, for the transmitter, this saw-toothed each train are in step with thosein the correwhich contai any known or suitable. arrangemitt" for integrating the variable width pulses andthus converting them into oscillations of speech frequency, andalso suitable terminal equipment for feeding these oscillations into the lines.

A preferred form. of apparatus will now be de scribed with reference to Figs. 58. The waveform of the transmitted signal is illustrated diagrammatically in Fig. 5. From this it is; clear that. the marker pulse has a duration or 22 microseconds and occurs at the end of the, eight speech channels; 1. e. the end of the mar er pulse in stead of its beginning marks the da um point in time. Each speech channel pulse has a duration or 4 microseconds when unmodulated; when] modulated by the speech currents its duration; can vary between 1 and 7 microseconds- The leading edge of each speech channelpulse is fixed and the position 01 the rear edge varies with the modulation, as in the system previously de- 1 scribed. The duration of the gaps, separating the various pulses is indicated in the figure audit is clear that any pulse will be separated i'roml its neighbour by a gap of at least 3 microseconds duration. The duration of each cycle is '111 microseconds corresponding to a repetition frequency of 9100 cycles per-second. l The pulse generating circuits are shown in Fig. 6. In order to simplify the drawing only one of the eight identical series of valves for generating the speech channel pulses is shown. In general, the operation of these circuitsis similar to that'of the circuits already described with reference to Figs. 2 and 3 the only important. difi'erence being the method of generating the marke pulses. The 9100 cycle repetition frequency is generated by the sine wave oscillator 20, the sinusoidal output 01 which is appliedto the control grid of the marker pulse generator 2|, and also to the control grid of the saw-tooth voltage generator'fl. As illustrated in curve I of Fig. 7 the grid 01' the valve 21 is negatively biased so that only the peak of the positive halt-cycle of the applied voltage is effective in removing the cut-off bias and rendering the valve conductive, .the duration of the conductive period being 22 microseconds. Consequently a negative pulse of 22 microseconds duration appears in the. anode,

circuit (curve II, Fig. 7) and this is applied to the outer grid of the common mixer valve 23. A

1 erator for'the first speech channel.

(curve III, Fig. 7). During this period the convalves 28, 29 and 30 constitute the pulse gen- The opera -tion of this-.is substantially similar to that-al- 1 ready described with reference to Figs. 2 and 3 and need only be outlined briefly. As before, the timing of the pulses is determined by an appropriate choice of the cathode potential of the 1 valve 21, this being fixed by connecting the cathode to the first tapping point on the potentiometer 4'. The negative impulses which occur in the anode circuit of the valve 21 each time it 1 conducts are applied through the variable condenser 3| and resistance 32 to the control grid jof the valve 28. The duration of the negative 1 impulse applied to this control grid can be varied by varying the capacity of the condenser 3|, and

in thiscase is set at approximately '7 microsecj OIidS. valve 28 is cut off, the condenser 33 charges and then suddenly discharges at the end of the period During this period of 7 microseconds the thus producing in the anode circuit of the valve 23, a positive pulse ofsaw-toothed form lasting jfor.7 microseconds, This pulse is applied tothe gcontrolgrid of the modulator valve 29 and drives the grid positive with respect to the cathode for 1a period the length of which depends upon the cathode potential. In the absence of a'modulating voltage the cathode potential is such that the grid is driven positive for a. period of 4 microseconds. The speech currents for channel I are fed from the incoming line into a conventional hybrid circuit 34, and after amplification by the valve 30 are injected into the cathode circuit of the valve 29 to vary the cathode potential in ac- :cordance with the speech current amplitude. As a result the grid of valve 29 is driven positive for a period varying between 1 and 'l microseconds according to the speech amplitude, and corresponding negative pulses appear in the anode circuit of valve 29. These are fed to the control grid of the common mixer valve 23, together with the outputs of the similar pulse generators for the seven remaining channels. The control grids of the first valvesof these generators are all fed vrith the saw-toothed voltage from the output valve 26 whilst their cathodes are connected to appropriate taps on the potentiometer 4. The

output of the mixer valve 23 will then consist of a series of positive pulses interlaced in the manner shown in Fig. 5, and after amplification by the valve 35, this output is employed to modulate the amplitude of an ultrahigh frequency carrier generated by the transmitter 36.

i The separating and demodulating circuits are shown in Fig. 8. The pulse modulated carrier is received by a supersonic-heterodyne receiver 4!,

the output from the second detector being applied to the-control grid of a limiting valve 42. The negative bias applied to this grid results in the lower portion of the pulses being cut-off, whilst the load in the anode circuit of the valve sets an upper cut-ofi limit. As a result only a. thin horizontal slice" of the pulses is transmitted to the control grid of the cathode follower stage 43. The output of this stage is fed through the resistance 44 to the control grid of the valve 45.

being limited in thiscase to 14 microseconds 1 of the valve 45 to the control grid of a saw-toothed During each [pulse the diode 46 shunting the resistance 44 is cut off, and the small condenser 41 is charged through the resistance 44 until the diode suddenly conducts at the end of the pulse. The voltage on the grid of the valve 45 will thus increase during each pulse and themsuddenly fall at the end of the .pulse, and since the duration of a marker pulse greatly exceeds that of the speech channel pulses, it will reach amuch higher value at the end of each marker pulse. Corresponding voltage changes are fed from the cathode circuit voltage generator 48 having the usual charging resistance 49 in its anode circuit and condenser 50 connected across it. The voltage changes due to the speech channel pulses are insuflicient to swing the control grid positive and consequently the condenser 50 charges until the arrival of a .marker pulse produces a voltage change of sufficient amplitude to render the valve 48 conductive. There is thus produced in the anode circult of the valve 48 a saw-toothed voltage which is synchronous with that produced by the valve 22 of Fig. 5, and which starts increasing at the end of amarker pulse and flies back toits original value during the marker pulse. Thiss'awtoothed voltage is fed via the cathode-follower stage 5| to the control grid of the'valve 52. This valve together with the valve 53 constitute the generator of the stroboscope pulses for the first channel. They operate in manner substantially v the marker pulse.

similar to the valves 21 and 28 of Fig. 6, except that as the valve 53"has no condenser corresponding to condenser 33 of Fig. 5 connected across it, the positive stroboscope pulses in its anode circuit will be square shaped instead of saw-toothed. The timing of. these pulses is arranged to coincide with that of the speech pulses of the first channel by connecting the cathode of valve 52 to the appropriate tap on the potentiometer 40, whilst the width of the pulses is fixed at approximately 9 microseconds by choosing an appropriate setting for the condenser 58. The stroboscope pulses are applied to the inner grid and'the signal pulses from the cathode circuit of valve 43- are applied to the outer grid of the separator valve 54. This operates in the manner already described in connection with Fig. 4, and only the speech pulses of the first channel appear in its anode circuit. These are applied to an integrating circuit comprisinga low-pass filter 55 and an output amplifier 56 which act is known manner to ,produce an output voltage varying in amplitude in accordance with the original speech current. This is applied via terminals 5'! to the hybrid circuit 34 and from thence to the outgoing line. Series of valves, each similar to the series 52-56, are employed for each of the remaining seven speech channels, the timing of the str0bo scope pulses for each channel being arranged to coincide, with the speech pulses in that channel by connecting the cathode of the first valve to the appropriate tap on the potentiometer 40.

As mentioned above, the most important difference between the arrangement shown in Figs. 6 and 8 and the system described with reference to Figs. 1-4 resides in the method of generating With this method more satisfactory stability of the synchronization in the receiver can be obtained.

Inspection of the pulse widths, the spaces between thein, and the position of the stroboscope pulses relative to their respective speech channel ingly employed at the receiver. For thispurpose plurality oi incoming signal channels, a signal pulse generator for each channel, means for gencrating a periodically recurring triggering voltage the repetition frequency can be divided to produce a, suitable operating frequency for 'synchronous motors at the transmitter and receiver foroperatlng the enciphering and deciphering switches. Y

The system according to the invention the following advantages as compared with multichannel systems depending on the use of tuned filters. The equipment is much lighter and smaller and requires less skill in construction, since, in the main, it uses valves with simplercondenser-resistance coupling networks; Also the circuit constants for each channel are identical,

selection of a channel being determined simply 3 by the adjustment of a potentiometer. Furthermore. the poweravailable'is (approximately) in versely proportional to the number-of channels i 15. periodically recurring triggering voltage arranged to trigger each selecting impulse generator in v generator, a selecting circuit for each channel arranged to trigger each generator in turn at a fixed repetition frequency, a device associated with each generator for varying the duration of the pulses in accordance withthe amplitude 01 the; signal in the corresponding channel, a com- '-mon output. circuit inwhich all the trains of pulses from all said generators are interlaced,

1 radio transmitting and receiving circuits for transmitting the interlaced pulses over a single link and reproducing them at a receiving point, a selectin impulse generator for each channel Iatfsaidreceiving point, means for generating a synchronism with the correspondingsignal pulse fed with the interlaced signal pulses and with the output of one of said selecting impulse gen- -erators and arranged to, pass one only of the interlaced trains of signal pulses, and an outgoing instead of to the square of the number elf-charinels. Finally, by' using the techniq es: pulsed transmission it 'ispossibie; by using suitable limiterstorecelveonly' thin horizontal slice oi-all the pulses, thus eliminating or reducing noise "and the effects of any'fading which mayioccuriq on the other hand a band pass of several megacycles 7 per second is required by the system which is therefore, applicable only to radio links operating,

on very shortwave-lengths or to routes usinghigh uu' m c l we claim:

' 1. A method r, transmitting electricali signais from a number of channels over, a single link to a receiving point and there separating'anddistributingthe'm'to an equal number or channels,

which comprises the steps of generating at-the j channel from each selecting circuit.

3. An electrical signalling system comprising a .pluralityoi incoming signal channels, a signal I a'pu'lse-generator for each channekmeans 'iorgencrating a periodically recurring triggering voltage .arranged to trigger each generator in turn at a 30 ilxedrepetition frequency, :a device associated with each generator for varying the duration "of.

. the pulsespin accordancewith the amplitude or the signal in the corresponding-channel, a generat'orfor producing-marker pulses of'longer duration'than the sl'gnal pulses -atsaid fixed repetition; "irequencmacommon output circuit in which all theitrains oi'pulsesf-rom all said generators are interlaced,'-radio transmitting and-receiving'circults for'transmittingxtheinterlaced pulses-over a single link and reproducing them at a receiv lug-point, a discriminating circuit for separating said marker pulses from the-interlaced channel pulses,' at said receiving-point, a selectingiimpulse generator foreach channel, means under the controloi 'the separated marker'pulses ior transmitting point a separate train oi' voltage pulses for each channel,- the repetition'irequencyl oi the pulses being the same for all the trains but the timing oi the pulses in eachtraingbeing diil'erent, varying the duration of the pulses in each train in accordance with variations mam-4 plitude .oi the signal'irom the 1 correspondin 1.

channel, generating a train of marker pulses having a, repetition frequency equal to that oi the channel pulses but an appreciably longer duration, interlacing the various trains of pulses, utillzing the interlaced pulses to modulate a radio frequency wave, transmitting the radio frequency. wave to a receiving point, demodulating theradiofrequency wave to reproduce theinterlaced pulses at the receiving point, separating the 4 marker pulses from the interlaced channel pulses, .utiliz-". ing the marker pulses to control the generation-3' ot a separate train of selecting impulses of fixed duration i'or each channeLthe timing ofthe selecting impulses in each train-being the same as that 01' the pulses in the corresponding'train,

combining in a separate circuit for each'channel the interlaced pulses with the selecting impulses of one 01' the trains, utilizing the selecting impulses in each of said circuits to select the pulses generating a periodically recurring triggering 1 voltage, arrangedto trigger each'generator-"in synchronism with the corresponding signalpulse generatoni-a selecting circuit for each channelled with-the interlaced signal pulses and with the output of one 01' said selecting impuls'egenerators and arrangedto pass one only or the interlaced trains of signal pulses, and an outgoing channel from each selectingcircuit. 1 1

4. An electricalsignalling system according claim 2 wherein there is provided: a generator for devehpmg i f v ltage recurrin at the: fixed repetition irequency' and wherein 4 h each signal pulse generator includes a-thermionic valve having its cathode potential fixed at a voltage which is positive relative to its grid p tential,- ',this voltage being diflerent for each generator, 'means for applying-the saw-toothed voltage to the grid of all the thermionic valves to causeeach one to conduct in turn. when its changinggrid potential equals its cathode potential, and means for utilizing the sudden conduction to develop a in step therewith and to suppress the remainder,

and converting the selected pulses from each circuit into a signal of varying amplitude.

2. An electrical signalling system comprising a 5. .An electrical signalling system according to claim 2 wherein the device for varying the duration of the signal pulses from each generator comprises a first thermionic valve having a negative bias on its control grid and a condenser connected in shunt therewith, a connection for applying the signal pulses to the control grid of suppressor grid, each negatively biased, means for applying the interlaced signal pulses of positive voltage to one grid and the positive selecting impulses from one generator to the other grid, whereby the valve conducts only when a selecting impulse and a signal pulse are simultaneously present.

7. Anelectrical signalling system according to .claim 3 wherein the discriminating circuit in-' cludes a condenser arranged to be charged during each incoming pulse to a potential dependent upon the duration of the pulse and to be discharged at the end of the pulse, a thermionic valve having a negatively biased control grid,

and a connection for applying the'condenser voltage to the control grid of said thermionic valve, which thereby conducts to a greater extent during a marker pulse than during a signal pulse.

8. An electrical signalling system comprising a plurality of incoming signal channels, a signal pulse generator for each channel, a marker pulse generator, means for generating a first sawtoothed voltage wave having a fixed repetition frequency, a thermionic valve in each generator responsive to said saw-toothedvoltage wave and arranged to conduct at a certain point along the wave which is different for each generator, means in each generator for utilizing the conduction to develop a pulse, means in each signal pulse generator for varying the duration of the pulse in accordance with the instantaneous amplitude of the corresponding signal, an output circuit in 1 which the pulses from all the l interlaced, radio transmitting and receiving cir- 1 cults for transmitting the interlaced pulses over 3 a single link and reproducing them at a receiving 5 point, a discriminating circuit-for separating the marker pulses from the interlaced signal pulses generators are at said receiving point, meansunder the control 01' the separated marker pulses for generating a. second saw-toothed voltage wave synchronous 12 with the first, a selecting impulse generator for each channel responsive to said second sawtoothed wave and operating in the same manner as the corresponding signal pulse generator to develop impulses of fixed duration in synchronism with the corresponding signal pulses, a selecting circuit for each channel fed with the interlaced pulses and W'iththe selecting impulses from one of said generators and arranged to pass one only of the interlaced trains of pulses, and an outgoing channel for each selecting circuit.

9. An electrical signalling system comprising a plurality of incoming signal channels, a signal pulse generator for each channel, a sine wave oscillator, means controlled by said oscillator for developing a first saw-toothed voltage wave, a marker pulse generator controlled by said oscillator and arranged to develop a pulse during the flyback of said saw-toothed wave, a thermionic valve in each signal pulse generator responsive to said saw-toothed voltage wave and arranged to conduct at-a certain point along the wave which is different for each generator, means in each signal pulse generator for utilizing the conduction to develop a pulse, means in each signal pulse generator for varying the duration of the pulse in accordance with the instantaneous amplitude of the corresponding signal, an output circuit in which the pulses from all the gen erators are interlaced, radio transmitting andat a receiving point, a discriminating circuit for separating the marker pulses from'the interlaced signal pulses at said receiving point, means under the control of the separated marker pulses for generating a second saw-toothed voltage wave synchronous with the first, a selecting impulse generator for each channel responsive to said second saw-toothed wave and operating in the same manner as the corresponding signal .pulse generator to develop impulses of fixed duration in synchronism with the corresponding signal pulses, a selecting circuit for each channel fed with the interlaced pulses and with the selecting impulses from one of said generators and arranged to pass one only of the interlaced trains of pulses, and an outgoing channel for each selecting circuit.

WILLIAM ALAN STEWART BUTEMENT, ALAN JOHN HENRY OXF'Q 

